This invention pertains generally to phased array antennas for radar and in particular to antennas of such type wherein antenna elements are connected together to form a monopulse antenna with linear polarization.
It has been recognized that when a monopulse antenna with linear polarization is used in a guided missile during flight to determine the direction from which radio frequency signals arrive, the effect of any type of curved radome on the polarization of such signals may introduce unacceptably large error in the determination of such direction. Any type of radome now known subjects radio frequency signals passing through to "depolarization," meaning that the polarization of the radio frequency signals incident on the radome is altered. At best the effective amplitude of the radio frequency signals impinging on the antenna elements will be attenuated; at worst the direction indicated by the monopulse nulls will also differ from the true boresight direction of the monopulse antenna. In the worst condition, depolarization due to the effect of any practical radome may cause an unacceptably large error in the determination of direction to a target.
The problem is made almost insoluble when a so-called "cross-polarization" jammer is actuated. In such a situation the depolarization effect of the radome converts enough of the cross-polarized jamming signal to a signal having a polarization accepted by the monopulse antenna to cause an excessively large shift in the monopulse nulls, with the result that a correspondingly large error in the indicated direction of such a jammer is experienced.